Aligned carbon nanotube-polymer materials, systems and methods

ABSTRACT

The invention is directed to carbon nanostructure composite systems which may be useful for various applications, including as dry adhesives, self-cleaning applications, electronics and display technologies, or in a wide variety of other areas where organized nanostructures may be formed and integrated into a flexible substrate. The present invention provides systems and methods wherein organized nanotube structures or other nanostructures are embedded within an adhesive, with the properties and characteristics of the nanotubes or other nanostructures exploited for use in various applications. In one aspect, the invention is directed to a self-cleaning carbon nanotube composite material that includes a substrate, an adhesive coating on at least a portion of the substrate, a plurality of carbon nanostructures formed into a predetermined architecture, each of the plurality of nanostructures having a substantially predetermined width and length, and the architecture of the plurality of nanostructures defining at least one orientation for a plurality of nanostructures, and defining the approximate spacing between nanostructures and/or groups of nanostructures, the carbon nanostructures architecture being at least partially adhered to the adhesive coating on the substrate in a manner that the architecture is stabilized in the predetermined architecture, wherein the carbon nanostructures architecture renders the composite material superhydrophobic.

This patent application is a continuation-in-part patent applicationclaiming priority to U.S. patent application Ser. No. 11/428,185 filedon Jun. 30, 2006 and U.S. patent application Ser. No. 11/675,442 filedFeb. 15, 2007 which is a continuation-in-part of U.S. patent applicationSer. No. 11/428,185 filed on Jun. 30, 2006 and U.S. Provisional PatentApplication Ser. No. 61/114,482 filed Nov. 14, 2008 which is acontinuation-in part of U.S. patent application Ser. No. 11/428,185filed on Jun. 30, 2006, which are incorporated herein by reference intheir entirety.

FIELD OF INVENTION

The present invention relates to composite carbon nanostructures, suchas nanotubes, adhered to an adhesive for providing diverse systems fordifferent applications, such as dry adhesives, self-cleaningapplications, electronic systems, display devices and otherapplications. The invention is also directed to methods for forming suchcomposite materials and for their use, including materials and productshaving self-cleaning properties.

BACKGROUND OF THE INVENTION

The use and development of carbon nanotubes has expanded, as thesematerials have shown to be valuable in next generation industriesincluding the fields of electronics and chemistry. The furtherdevelopment of carbon nanotube technology allows organized structures orintertwined randomly oriented bundles of carbon nanotubes to be formed.Techniques have been developed to controllably build organizedarchitectures of nanotubes having predetermined orientations, such asvertically aligned nanotubes. Although such structures may be useful fora variety of purposes, the structures by themselves may be limited interms of function and application.

In the area of adhesives for example, it would be desirable to providedry adhesives which may be useful in a variety of applications andenvironments for which standard adhesives have deficiencies. Adhesivesare typically wet and polymer-based, and have low thermal and electricalconductivity. For electronics, micro-electro-mechanical systems (MEMS),low or zero atmosphere environments, cryogenic or high temperatureenvironments, or a variety of other areas, it would be desirable toprovide a dry adhesive which is selectively attachable and detachableto/from a surface. It would also be desirable to provide an adhesivewhich has other beneficial properties, such as high electrical andthermal conductivity or high adhesion strengths while being selectivelydetachable. For example, the mechanism which allows a gecko lizard toclimb a vertical surface or any other surface is based upon the anatomyof the gecko's feet and toes, wherein each five-toed foot is coveredwith microscopic elastic hairs called setae. The ends of these hairssplit into spatulas which come into contact with the surface and induceenough intermolecular (van der WAALS, [VdW]) forces to secure the toesto the surface. The gecko's foot anatomy allows them to selectivelyadhere to any surface which they touch. Although attempts have been madeto provide synthetic systems which mimic the gecko's feet and toeanatomy, no such systems have generally been successful. It would bedesirable to provide an adhesive which mimics these characteristics, andprovides a surface which interacts with other surfaces viaintermolecular or VdW forces, via nanostructure technologies.

It has also been noted that the external surfaces of many plants andanimals have a rough surface structure combined with an ideal surfacechemistry to create self-cleaning, water-repellant surfaces. Forexample, the VdW interaction between the hairs of the gecko and asubstrate after contact may play a role in self-cleaning. Additionally,self-cleaning characteristics are found on the leaf surface of the N.nucifera (the white lotus) and the wing surface of many insects, whichcombine a topology describing a high degree of surface roughness with achemistry that exhibits low surface energy. Such a combination creates asuperhydrophobic surface that sheds liquids of various types, and allowsparticulates to be removed when subjected to an external force such asrolling water droplets, or flowing air. It would be desirable to provideself-cleaning characteristics in association with adhesive typematerials. Some other systems found in nature that exhibit self-cleaningproperties include the leaves of the lotus and lady's mantle plants. Ithas also been found that the For example, the self-cleaningcharacteristics found on the leaf surface of the N. nucifera (the whitelotus) and the wing surface of many insects combine a topologydescribing a high degree of surface roughness with a chemistry thatexhibits low surface energy thus creating a superhydrophobic surfacesuch that it sheds liquids of various types, allowing particulates to beremoved when subjected to an external force such as rolling waterdroplet, or flowing air. The surface of the lotus leaves for examplehave two levels of microscopic roughness, which along with a hydrophobicwax coating, render the lotus leaves superhydrophobic. A water droplet,when placed on the surface of a lotus leaf, forms a large contact anglewith low contact angle hysteresis. This results in the water dropletsrolling off the leaf surface, leaving the surface clean. Leaves of alady's mantle plant have hairs of 10 μm and a length of 1 mm. It hasbeen noted that the individual hairs may be hydrophilic. However, whenacting together on the surface, they make the surface of the leavessuperhydrophobic. It would be desirable to provide self-cleaningcharacteristics in association with adhesive type materials, as well asother materials or products.

In a variety of other areas, the use of organized carbon nanostructuresin unique configurations may provide valuable functions in self-cleaningadhesives, biocompatible or bioactive systems, electronic displays,functional films or skins, or other applications.

SUMMARY OF THE INVENTION

The present invention is therefore directed to carbon nanostructurecomposite systems which may be useful for various applications,including as dry adhesives, electronics, self-cleaning applications anddisplay technologies biosystems, or in a wide variety of other areaswhere organized nanostructures may be formed and integrated into aflexible substrate.

The present invention provides systems and methods wherein organizednanotube structures or other nanostructures are embedded withinpolymers, adhesives or other flexible materials to provide a flexibleskin-like material, with the properties and characteristics of thenanotubes or other nanostructures exploited for use in variousapplications.

In one aspect, the invention is directed to a self-cleaning carbonnanotube composite material. The composite material includes asubstrate, an adhesive coating on at least a portion of the substrate, aplurality of carbon nanostructures formed into a predeterminedarchitecture, each of the plurality of nanostructures having asubstantially predetermined width and length, and the architecture ofthe plurality of nanostructures defining at least one orientation for aplurality of nanostructures, and defining the approximate spacingbetween nanostructures and/or groups of nanostructures, the carbonnanostructures architecture being at least partially adhered on thesubstrate in a manner that the architecture is stabilized in thepredetermined architecture, wherein the carbon nanostructuresarchitecture renders the composite material self-cleaning upon theapplication of a fluid to the material.

Another aspect of the invention is directed to a method of forming aself-cleaning carbon nanotube composite material. The method includesthe steps of providing a substrate having a predetermined configuration,providing a plurality of carbon nanostructures formed in a substantiallypredetermined architecture supported on the substrate, at leastpartially embedding the plurality of carbon nanostructures in apolymeric material in a manner to stabilize the predeterminednanostructure architecture at least partially therein, wherein thearrangement of the predetermined nanostructure architecture renders thecomposite material self-cleaning upon the application of a fluid to thematerial.

A further aspect of the invention is directed to a method of forming aself-cleaning carbon nanotube composite material. The method includesthe steps of forming a composite material having a predeterminedhierarchial architecture of substantially vertically aligned carbonnanostructures embedded in a polymeric material, the polymeric materialhaving a predetermined thickness, wherein the hierarchial architectureincludes structures between 50 to 500 microns in width integrated withstructures formed in the range of 1 to 40 nm. The spacing between thesubstantially vertically aligned carbon nanostructures is substantiallypredetermined and the carbon nanostructures extend from at least onesurface of the polymeric material a substantially predetermined amount,such that the exposed carbon nanostructures renders the compositematerial self-cleaning.

The invention is also directed to methods and systems wherein apredetermined architecture of carbon nanotubes are constructed on asurface that are superhydrophobic, thus providing the composite materialwith self-cleaning properties. In examples, the invention is furtherdirected to forming carbon nanotubes into a predetermined architecturesupported on a substrate to have a topology describing a high degree ofsurface roughness with a chemistry that exhibits low surface energy.Such examples thus create a surface such that sheds liquids of varioustypes, and allows particulates to be removed when a fluid as appliedthereto, such as when subjected to an external force such as rollingwater droplet, or flowing air. Such self-cleaning characteristics may beformed in association with dry adhesive type materials.

Other aspects and advantages of the invention will become apparent upona reading of the description of the present invention in conjunctionwith the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microscope image of vertically alignedmulti-walled carbon nanotube structures.

FIG. 2 is a schematic illustration of a method for preparing a carbonnanotube-polymer composite according to the invention.

FIG. 3 is a schematic illustration of an alternative method forpreparing a carbon nanotube-polymer composite according to theinvention.

FIGS. 4A-4F show scanning electron microscope images of the carbonnanotube structures embedded in a polymer matrix and having a portionthereof exposed from the surface.

FIGS. 5A-5C show graphical representations of adhesion characteristicsof a carbon nanotube/polymer composite material formed in accordancewith an embodiment of the present invention, showingdeflection-versus-displacement curves during loading-unloading cycles ofa silicon probe engaging exposed carbon nanotubes associated with thecomposite as formed according to FIG. 2 or 3 as examples.

FIG. 6 is a schematic illustration of a further method of forming carbonnanotube-polymer composites according to the invention.

FIG. 7 shows a scanning electron microscope image of the carbon nanotubearchitectures before being embedded into a polymer matrix.

FIG. 8 is a scanning electron microscope image showing the nanotubearchitectures of FIG. 7 after polymer infiltration.

FIG. 9 is a top view of nanotube walls prior to polymer infiltration.

FIG. 10 shows a cross-sectional scanning electron microscope image ofthe nanotube walls shown in FIG. 9 after polymer infiltration.

FIGS. 11A and 11B show graphs of electrical resistance of ananotube-polymer composite structure such as formed according to FIG. 6,relative to strain and compression.

FIG. 12 shows a schematic illustration of a flexible Field EmissionDisplay (FED) using carbon nanotube/polymer composites according to theinvention.

FIG. 13 shows a Fowler-Nordheim plot of field emission, with the insetgraph showing emission current for applied voltages for severalnanotube-polymer composites according to the invention.

FIG. 14 shows a schematic illustration of a bioactive composite for usein a biological system formed according to an embodiment of the presentinvention.

FIG. 15A is a scanning electron microscope image showing the nanotubearchitectures of synthetic setae made of micro-patterned carbon nanotubebundles of about 500 μm.

FIG. 15B is a scanning electron microscope image showing the nanotubearchitectures of synthetic setae made of micro-patterned carbon nanotubebundles of about 50 μm.

FIG. 15C is a scanning electron microscope image showing a highermagnification of the nanotube architectures of FIG. 15A.

FIG. 16 is a scanning electron microscope image showing thesuperhydrophobic behavior of micro-patterned carbon nanotubearchitectures by supporting a 10 μL water droplet.

FIG. 17 is a top view of a scanning electron microscope image of FIG.16.

FIG. 18A is an optical image of a clean micro-patterned carbon nanotubearchitecture.

FIG. 18B is an optical image of a soiled micro-patterned carbon nanotubearchitecture.

FIG. 18C is an optical image of a soiled micro-patterned carbon nanotubearchitecture cleaned with water.

FIG. 18D is an optical image of a soiled micro-patterned carbon nanotubearchitecture cleaned with mechanical vibration.

FIG. 19A depicts an apparatus used to measure shear data ofmicro-patterned carbon nanotube architectures.

FIG. 19B is a graph charting the shear measurements for a control, asample soiled and subsequently cleaned through the application ofmechanical vibrations, and a sample soiled and subsequently cleaned withwater.

FIG. 20 is a graph charting the shear measurements for a single-wallcarbon nanotubes configuration according to an example of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the FIGS., a first embodiment of a carbonnanostructure/polymer composite material according to the invention willbe described with reference to FIGS. 1-4, wherein a large number ofcarbon nanostructures 10, such as multi-walled carbon nanotubes (MWNT),are formed on a substrate 12 as shown in FIG. 1. The growth ofvertically aligned carbon nanotubes 10 on a substrate 12, such assilicon substrate, may be performed in any suitable manner, with variousapproaches having been developed. Alternatively, the nanostructures maybe single-walled nanotubes, or nanosheets or other nanostructures formedof carbon.

One method is to selectively grow carbon nanotubes on silica templateslocated on a silicon substrate at set forth in Z. J. Zhang, B. Q. Wei,G. Ramanath, P. M. Ajayan, Appl. Phys. Lett. 77, 3764 (2000), which ishereby incorporated herein by reference. The use of this templatestructure is advantageous in that it does not require the deposition andpatterning of a catalyst material on the substrate 12, although such anapproach may also be used according to the invention. Another method maybe as described in published U.S. Patent Application 2003/0165418, whichis incorporated by reference herein. Any other suitable methods toprovide organized architectures of carbon nanotubes on a substrate arealso contemplated and within the scope of the invention. The substrate12 may be formed of other materials such as quartz, molybdenum, or othersuitable materials. Further, the carbon nanostructures may be formed byother suitable techniques, such a by plasma enhanced chemical vapordeposition, or any other suitable technique. Such processes may formother carbon nanostructures, such as nanofibers, sheets, pillars orother forms. The carbon nanotubes or other carbon nanostructures alsohave good mechanical properties such as very high Young's modulus andvery high tensile and bending strengths, making them useful for theapplications as described herein.

Turning now to FIG. 2, a first method for producing a carbonnanotube-polymer composite according to the invention is shown. In thisexample, the growth of nanotubes on a silicon substrate may beaccomplished through chemical vapor deposition (CVD). A gaseous mixtureof ferrocene (0.3 g), is used as a catalyst source, and xylene (30 mL),is used as a carbon source. Other suitable materials may be used ifdesired. The gaseous mixture is heated to over 150° C. and passed overthe substrate 12 for ten minutes, with the substrate 12 itself beingheated to approximately 800° C. in a quartz tube furnace. The substrate12 may be provided with an oxide layer 13 on which MWNT 10 grow withcontrolled thickness and length. If desired, the oxide layer of thesubstrate 12 can be patterned by photolithography or other suitabletechniques, and may be followed by a combination of wet and/or dryetching in order to create various predetermined patterns of the oxidelayer 13 and correspondingly of the carbon nanotubes 10 grown thereon.After the oxide layer 13 of the substrate 12 is covered with MWNT toform a desired and predetermined architecture of carbon nanostructures,the sample, with the MWNT side facing up, has at least one polymericpolymer precursor material, such as at least one monomer, poured thereonto encase the carbon nanostructure architecture. Thereafter, theprecursor materials are polymerized to embed the carbon nanostructuresin a polymer matrix. For example, a methyl methacrylate monomer (60 mL)may be used, and then polymerized using a 2,2′-azobis(isobutyronitrile)initiator (0.17 g) and a 1-decanethiol chain transfer agent (30 μL) in aclean room. As seen in FIG. 2, the MWNT are then encapsulated within apolymer matrix 14 on substrate 12. The MWNT or other nanostructures andarchitectures are embedded and stabilized in the PMMA matrix 14, withoutdisruption of the organized architecture of the carbon nanostructures asoriginally grown or provided on substrate 12. To facilitate maintainingthe desired architecture of the carbon nanotubes or othernanostructures, the introduction of the monomer or other precursors isperformed in a manner to flow around the nanotube architecture withoutforcing the nanotubes together or otherwise significantly disruptingtheir position and orientation on the substrate 12. Similarly,polymerization is completed without disruption of the nanotubearchitecture, with properties controlled to maintain the desiredconfiguration. For example, polymerization may be performed in a mannerto reduce possible effects of evaporation upon the matrix 14 andultimately the carbon nanotubes embedded therein. After completion ofpolymerization in a water bath at 55° C. for 24 hours, the matrix 14 maysimply be peeled from the substrate 12 forming a flexible skin-likesheet in which the carbon nanotubes 12 are fully or partially embeddedand stabilized.

Polymeric matrix materials according to the invention may be of anysuitable type, wherein polymeric polymer precursors may includemonomers, dimers, trimers or the like. Monomers utilized in thisinvention may generally be selected from the family of vinyl monomerssuitable for free radical polymerization under emulsion conditions.Non-limiting examples of suitable vinyl monomers include methacrylates,styrenes, vinyl chlorides, butadienes, isoprenes, and acrylonitriles,polyacrylic and methacrylic esters and any other suitable precursormaterials. The matrix polymer may be a polymer of one or more of thefollowing monomers: methyl methacrylate (MMA), other lower alkylmethacrylates (e.g. ethyl methacrylate, propyl methacrylate, isopropylmethacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, etc., as anexample. A starting monomer formulation may also include one or morepolymerization initiators. These include, for example, benzoyl peroxide,lauryl peroxide, azobis(isobutyronitrile), 2,2′-azobis(2,4-dimethyl-4methoxypropionitrile), and 2,2′-azobis(2-methylpropionitrile) or othersuitable initiator materials. These are used in small amounts which arewell known in the art. Any initiator that is suitable for free radicalpolymerization can be considered according to the invention. Further,the polymer matrix may also be modified using nanofillers as an example.Nanofillers are fillers having at least one dimension in the nanoscale(1-999 nm). Suitable fillers may include, without limitation, clayminerals, fibers, micro-spheres, and layered silicates. Such nanofillersmay have their surfaces modified by surface functionalization with ionicgroups or the like to provide desired interaction in the polymer matrix.Additional optional components may be present in the polymer matrix ifdesired, such as chain transfer agents, which are typical of freeradical polymerizations, to facilitate the polymerization of the monomeror other polymerizable components. Other optional components that mayfacilitate use in various applications may include colorants,mold-release agents, and other known modifiers. The starting monomerformulation or mixture may also include a crosslinking agent, as forexample ethylene glycol dimethacrylate or other difunctional (i.e.,diolefinic) monomer or mixture thereof. The polymeric materials may alsobe thermoset plastics or other suitable epoxy type materials. Epoxyresins useful in the present invention can be monomeric or polymeric,saturated or unsaturated, aliphatic, cycloaliphatic, aromatic orheterocyclic, and they can be substituted if desired with othersubstituents besides the epoxy groups, e.g., hydroxyl groups, etherradicals, halogen atoms, and the like. Also, as will be described inrelation to other embodiments, materials such as silicones may be usedto integrate carbon nanostructures therein, such aspoly(dimethylsiloxane) or PDMS. Many other suitable polymeric or plasticmaterials are contemplated as will be understood by those skilled in theart. The materials of the invention may also be used to make non-wettingplastic surfaces that are self-cleaning.

The invention may be used to form adhesive type of devices or materials,wherein the carbon nanotubes are partially embedded and stabilizedwithin a flexible substrate or matrix. The methods for forming such adevice or product include a variety of suitable approaches, and othersuitable methods are contemplated. For example, the carbon nanotubes arepartially embedded and stabilized using a molten interface. In such anexample, a flexible material is heated to be molten, for example justabove its melting temperature, and vertically aligned carbon nanotubesare brought in contact with the material so that a predetermined,generally small fraction of tubes is introduced into the moltensubstance. Thereafter, the material is allowed to cool below its meltingpoint and the nanotube structures are partially embedded therewith.Another method may use a prepolymer, wherein a paste of prepolymer isprovided and carbon nanotube structures are brought in contact with thesurface so that only a part of carbon nanotube structure is inside theprepolymer. The prepolymer is then crosslinked, thus trapping one end ofcarbon nanotube structures in its matrix. Alternatively, a monomer,chosen such that it can be polymerized by applying at least onestimulant for example, may be used. Carbon nanotubes are brought incontact with the monomer and polymerization was set to occur. Onpolymerization, the polymerized material held the carbon nanotubestructures in place.

Another method may use a degradable substance. In this example, aprepolymer/monomer/solution/melt type of material, capable of degradingunder suitable conditions may be used to coat the carbon nanotubestructures such that they form a thin layer on the carbon nanotubestructures. Thereafter the matrix is exposed to suitable conditions thatwould degrade a portion of the substance thereby leaving exposed or openends of carbon nanotubes. A farther method may use a viscous material,wherein a thin layer of viscoelastic material is applied onto a flexiblesubstance and brought in contact with carbon nanotubes. The carbonnanotubes stick to the flexible substance via the viscoelastic material.

The flexible (or rigid) matrix 14 may then be used for a variety ofapplications, with one such application being to form a flexibleskin-like material which could be used as a dry adhesive, simulatinggecko foot-hairs. In this example, an adhesive tape or material may beformed using an array or architecture of nanotubes or nanostructuresformed in association with a flexible substrate. The nanostructures areformed in a hierarchical structure in which larger structures arecombined with smaller structures which together provide strong adhesioncharacteristics. As an example of a dry adhesive tape or material, theinvention provides for the transference of micro-patterned carbonnanotube arrays onto flexible polymer tape or material in a hierarchicalstructure. The dry adhesive tape or material according to an example cansupport a force (36 N/cm²), being nearly four times higher than theadhesion characteristics of the gecko foot, and sticks to a variety ofsurfaces, including materials such as Teflon.

As examples, as shown in FIGS. 4A-D, the hierarchical structure mayinclude micron-size structures formed by bundles of nanotubes ornanostructures 18, and nanometer-size structures 19 as shown in FIGS.4E-F formed by individual nanotubes or smaller bundles thereof. In thisembodiment, the nanometer-size structures 19 are integrated into thelarger micron-size structures 18. Such a hierarchical structure enhancesthe macroscopic adhesion characteristics of the tape or material, andfacilitates the translation of the weak van der Waals interactions ofthe structures into very strong aggregate attractive forces.

Flexible adhesive tapes are indispensable in people's daily lives, butadhesives using viscoelastic materials (wet adhesives) have variousdeficiencies, including degradation of the adhesion properties overtime. The stickiness of the wet adhesive is time and rate dependent, andsuch materials cannot operate in different environments such as undervacuum. The invention provides a dry adhesive tape type material, whichcan be formed in any desired configuration, for use in such applicationsas space applications. Further, the dry adhesive tape or materialaccording to the invention may be used for repeated attachment anddetachment applications. On coming in contact with any surface, thehierarchical structures formed of nanotubes deform, enabling molecularcontact over a larger area, and acting to translate the locally weak vander Waals (vdW) interactions into high attractive forces. Themulti-scale structures, including the micron-size structures formed bybundles of nanotubes or nanostructures 18, and the nanometer-sizestructures 19, may use micro fabricated multi-walled carbon nanotubes(MWCNT), but other suitable nanomaterials are contemplated.

The architecture of nanostructures includes both nanometer length scalesof structures in association with micrometer length scales ofstructures, which together achieve large macroscopic adhesion. Forexample, a cm2 area of the MWCNT patterns transferred on a flexible tapetype substrate was found to support 36 N. Similar adhesion forces areobtained on both hydrophilic (mica and glass) and hydrophobic (Teflon)surfaces. This dry adhesive tape or material according to this exampleof the invention show desirable adhesion and peeling properties andprovide a dry, conductive, reversible adhesive for use in a variety ofapplications such as microelectronics, robotics, space applications andmany others. FIGS. 4A-D show examples of aligned MWCNT, in SEM images,wherein the sizes of the micron-size structures formed by bundles orgroups of nanotubes or nanostructures 18 may vary from 50 to 500 micronsin width for example. Each of the micron-size structures 18, hasintegrated therein the nanometer-size structures 19 shown in FIGS. 4E-Ffor example. The structures 19 may comprise thousands of individualbundles of aligned MWCNT with an average diameter of 8 nm. Thenano-structures may be formed in the range of 1 to 40 nm for example, orwithin the range of 4 to 20 nm for example. The features or structures18 and 19 have shapes which may include, but are not limited to, square,circular, triangular and hexagonal cross-sections. The features orstructures 18 and 19 may thus have sizes may range from 5˜1000 μm, withspacing between structures which may range from 5˜500 μm as an example.The height of exposed CNT's may range from 10 μm˜1000 μm as an example.

For various adhesive applications, the use of MWCNT to construct thenanomaterial structures may provide desired characteristics due to thestrong nanometer-level adhesion that vertically aligned MWCNT materialsexhibit, along with excellent mechanical properties, electricalproperties and other properties. In the examples of FIGS. 4A-F, themicron scale structures 18 and nanometer scale structures werefabricated using a photolithography patterning process. A catalyst (Feand Al) was deposited on a silicon substrate in patterned patches withdimensions shown in FIGS. 4A-F. The MWCNT were grown at 700-800° C.using a mixture of ethylene and hydrogen gas and the MWCNT grew only inthe areas covered with the catalyst. The length of carbon nanotubes wascontrolled by the reaction time, and in these examples was around200-500 μm for the shown structures. The structures were thentransferred to a flexible substrate as previously described.

To measure the macroscopic adhesion forces, small areas of the formedflexible dry adhesive tapes or materials were pressed against a smoothmica sheet. The force to peel the tape or material off the mica surfacewas measured. In FIG. 5A, the values for adhesion forces associated witha tape or material having the hierarchical structures discussed aboveare shown for various pattern sizes. The adhesion characteristics werefound to be significantly higher than unpatterned nanotubes formed in aflexible substrate (not having the hierarchical structures). Further,with unpatterned nanotubes, the adhesion force is found to decrease withincreases in contact area, such that support of larger weights may notbe achieved by just increasing the contact area. To support largerforces, the hierarchical structure of micron size structures incombination with nanometer size features are found to avoid thispotential limitation. FIG. 5A shows the measured adhesion at zero degreeangle for the patterned CNT surfaces shown in FIGS. 4A-F by usinghierarchical patterned surfaces with width features of 50 and 500 μm,there was obtained a factor of 4-7 times higher adhesion forces ascompared to the unpatterned surfaces of similar area. The adhesionforces came out to be 23 N/cm2 which is 2-3 times higher than thenatural gecko foot-hairs for example. The advantages of hierarchicalpatterns became less prominent on reducing the patch size to 50 μm (and300 μm in height) because decrease in the ratio of the width to heightassociated with the micron size structures makes them more mechanicallyweak. If a smaller height is used, such as 50 μm structures 18 with aheight of 200 μm), an adhesion force of 36 N/cm2 was measured, being afactor of 4 times higher than the natural gecko setae for example. It isfurther found that the interfacial adhesion strength in these structuresare likely even stronger than that measured using this peeling geometry.The 50-500 μm patches of nanotubes made it possible to have the highforce/area for larger areas, with similar force/area values obtained fordry adhesive tapes or materials made with the hierarchical patterninghaving surface areas of 0.16 cm2 and 0.25 cm2 for example. As shown inFIG. 5B, the 50-500 μm structures 18 deform and behave independently,thereby increasing the adhesion characteristics and hindering crackgrowth by providing resistance to the propagation of cracks.

The adhesion characteristics of the dry adhesive tape or materialaccording to examples of the invention are also thought to be based upontranslation of the locally weak vdW forces to large attractive forces.Other forces on a microscopic scale may also be contributing, such ascapillary forces due to humidity in the environment for example. The dryadhesive tape or material is found to adhere to both hydrophilic andhydrophobic surfaces. For example, on hydrophilic surfaces such as micaand glass (water wets both surfaces), a partially hydrophobic surface(coated with polymethylmethacrylate that shows a water contact angle of70-80°) and a very hydrophobic surfaces (coated with poly(octadecylacrylate) comb polymer that shows a water contact angle of 110°).Measurements on rough Teflon surfaces also show large adhesion forces at0° angle that are comparable to those obtained for hydrophilic surfaces.Further, the adhesion based on vdW forces is time-independent as shownin FIG. 5C, distinct from a common viscoelastic based adhesive tape.

A further attribute of a dry adhesive tape (or other product) accordingto the invention is found in that the tape or material offers verylittle resistance when peeled from a surface at an angle. For example, adry adhesive tape or material using 500 μm features 18 is found to peeloff a mica substrate with an adhesive force of only 0.4 N/cm2 at 45°angle, 0.5 N/cm2 at a 30° angle, and 2.4 N/cm2 at a 10° angle. Thispeeling process resists any breaking or transfer of the MWCNT on thesubstrate and the dry adhesive tape or material can be reused many timeswithout damage. The energy of detachment (G) can be calculated usingG=F(1−Cos θ)/width, where F is the peeling force and θ is the peelingangle. This equation is valid for peeling angles greater than 45°, andthe elastic stiffness of the tape or material may facilitate at lowerpeeling angles. In examples according to the invention, G is 5 J/m2 onmica at a 45° peeling angle, being much larger than the thermodynamicwork of adhesion. On Teflon substrates, G=2.2 J/m2 at 45° peeling angleconsistent with the lower surface energy of Teflon in comparison tomica. The micron-size patterns 18 facilitate increasing the zone ofdeformation by bending the patterns and increasing the load bearingcapacity during peeling. The dry adhesive products according to examplesof the invention, using the hierarchical structures as described, withlength scales of microns and nanometers are found to achieve higheradhesion forces, with the ability to adapt to desired adhesioncharacteristics by increasing the area of the tape or other product.

It has also been found that the adhesion forces do not appear to beinfluenced by the chemical composition of the solid substrate in whichthe carbon nanotubes are partially embedded. The adhesivecharacteristics of the composite may thus provide uses as dry adhesivesfor microelectronics and low vacuum (space), cryogenic or hightemperature applications, where typical adhesives cannot be used, or ina variety of other environments or applications. It should also berecognized that devices made according to the invention may allowseparately formed composite materials according to the present inventionto interact and adhere with one another. Alternatively, thenanostructures may be exposed on both surfaces of the composite so as toallow each surface to act as an adhesive surface. Further, in this andother embodiments, the composite material may also yield significantbenefits in providing high thermal conductivity and/or insulationcharacteristics, which may be particularly important in electronics orother applications. The electrical conductance of the materials may alsoprovide the ability to utilize the composite materials in electricalapplications or to dissipate electrical energy.

In examples of the dry adhesive tapes or other products above, or inother examples, the ability to have portions of the nanotubes or othernanostructures exposed in a flexible matrix 14 enables interaction ofthe nanostructures with other surfaces or materials. As previouslyindicated, other methods for producing a composite having thesecharacteristics are contemplated, such as the use of a solvent etchingprocess to expose a portion of the MWNT 10, such as from thesubstrate-facing side of the matrix 14. For example, etching the top ofa formed composite with a solvent such as acetone or toluene andsubsequently washing with deionized water, will expose a predeterminedlength of the MWNT 10. The length of the nanotubes 10 which are exposedmay be controlled by varying the solvent etching time as an example.Other suitable techniques for selectively exposing a length of thecarbon nanotube fibers embedded within matrix 14 are contemplated aspreviously discussed. Thus, as should be evident, the particular natureof the exposed carbon nanotubes may be selectively controlled both upongrowing or producing the nanotubes themselves and forming thearchitecture on substrate 12 as desired, as well as in selectivelycontrolling the length and spacing of the exposed carbon nanotubes forvarious wide-ranging applications.

An alternative method for forming the composite according to theinvention is shown in FIG. 3. Again, a substrate 12 may havearchitecture of MWNT 10 grown thereon via an oxide layer or in any othersuitable manner. Thereafter, a flexible or rigid substrate 16 providedwith a partially cured polymeric material 14 disposed thereon is broughtinto contact with the MWNT 10 provided on substrate 12. In this example,the MWNT 10 are at least partially embedded within the partially curedpolymer 14, without disruption of the position or orientation of thecarbon nanotubes on substrate 12. The MWNT are only partially embeddedwithin the polymer matrix 14, but are fully stabilized in their originaland desired position as on substrate 12. Thereafter, the curing of thepolymer 14 is completed in a manner to maintain the position of thepartially exposed carbon nanotubes, and subsequent to curing, theassembly of the flexible or rigid substrate 16, and the polymer matrixincluding the partially embedded MWNT may be peeled from substrate 12.As shown in FIG. 3, a composite including the flexible or rigidsubstrate 16 and the polymer matrix 14 with partially exposed MWNT 10 isproduced, which again may be used for a variety of applications. Similarto the embodiment of FIG. 2, the nanotube configurations andarchitectures, as well as the length of the exposed nanotubes may beprecisely controlled for adapting to a variety of applications.

The ability to form a desired and predetermined architecture of carbonnanotubes or other nanostructures which are partially embedded andstabilized within a flexible polymer matrix according to the inventionmay also provide for flexible skin-like materials which may be used ascoverings or coatings in a variety of environments and applications. Forexample, electrodes may be formed having these composite materialsincorporated therein for significantly increasing the surface area orproviding other significant benefits. Additionally, it is possible toalign the carbon nanofibers and selectively expose a portion to formcoatings which may have significant properties such as anti-friction,anti-static, or non-wetting surfaces. For various embodiments, it may beuseful to form the nanostructure architecture such that individualcarbon nanostructures are tangled around adjacent carbon nanostructures(especially at their exposed ends). Such tangling may provide surfaceunevenness which may increase the surface area of contact with a surfaceor provide other benefits for certain applications. Alternatively, suchunevenness may be provided by forming the nanostructures to havedifferent heights among the various individual carbon nanostructures.

Although the above examples describe the use of multi-walled carbonnanotubes, other carbon nanostructures may be used in accordance withthe invention. Similarly, although the polymer used in the example isdescribed as a glassy PMMA, other polymers with different propertiessuch as, different glass transition temperature, crystallinity, moduli,flexibility and functionalities may be used for other applications ascontemplated in the invention. The ability to use various polymersprovides flexibility to tailor the flexible substrate produced for anydesired application. Chemical properties may also be controlled asdesired for various applications. Surface treatment may also beperformed to provide other characteristics and/or properties.

Turning now to FIG. 6, a further embodiment of fabricating a carbonnanotube array structure in conjunction with a polymeric substrate isshown. As a first step, similar to that previously described, aprepatterned silicon substrate 20 has bundles of MWNT grown thereonusing thermal CVD of Ferrocene and Xylene at 800° C., to form an arrayof vertically aligned patterned MWNT on the substrate at 22. In thisexample, a polymer, such as PDMS may be utilized for embedding the MWNTarchitectures so as to stabilize and fix the MWNT in position. A PDMSprepolymer solution, which may be a viscous mixture of base/curingagent, such as in a weight ratio of 10:1, may be poured over the alignednanotube structures on the substrate at 24. The arrays of nanotubes areembedded within the soft polymer matrix without disturbing the shape,size or alignment of the nanotubes. Although a particular architectureof aligned MWNT architectures is shown in this example, it should alsobe recognized that any desired architecture may be easily formed in asimilar manner. In this example, subsequent to embedding the nanotubestructures on the substrate within the soft prepolymer solution, anyexcessive polymer solution may be removed to obtain a desired thicknessfor the nanotube-PDMS composite film being produced. Again in thismanner, the particular thickness of the composite film may be controlledfor optimizing the arrangement for a particular application orenvironment (at 26). The PDMS is then thermally cured and, subsequently,self-standing nanotube-PDMS composite films may simply be peeled offfrom the silicon substrate at 28. This process has been used to makenanotube-PDMS composite films, wherein FIG. 7 shows a tilted SEM imageof an array of cylindrical pillars of selectively grown and aligned MWNTstructures on a substrate. In this example, the diameter of the MWNTstructures may be in the range of 1-500 μm using photolithography. InFIG. 8, the surface morphology of these nanotube pillars is shown afterPDMS infiltration, and subsequent completion of polymerization. It hasbeen found in this process, that it is possible to transfer smaller andmore densely distributed nanotube architectures into the PDMS matrix,such as structures only a few micrometers in scale. Further, thesestructures have been shown to effectively retain their originalalignment, shape, and size inside the resulting composite matrix, afterpolymerization and subsequent to peeling off from the substrate. Analternative nanotube structure is shown in FIG. 9, wherein nanotubewalls are grown on the substrate, wherein the walls may have desiredwidth, height and spacing between walls. In FIG. 10, a cross-sectionalSEM image of the nanotube walls shown in FIG. 9 show that thesestructures are retained subsequent to infiltration by the PDMS andpolymerization thereof. To facilitate this, the selected polymericmaterials may be chosen to provide conformal filling of the spacesbetween individual nanotubes and building blocks thereof in an effectivemanner. If desired, other chemical agents may be used or polymericmaterials chosen to enhance wettability relative to the nanotubearchitectures to form a relatively defect-free composite film retainingthe original nanotube architecture.

The composite films formed according to the invention provide anextremely flexible and otherwise deformable matrix which may allow thefilm to be produced in any of a variety of desired configurations andgeometries, while maintaining nanostructure architecture therein.Further, it was found that the composite films according to theinvention provide stable electromechanical structures, which enableuseful electronic applications. The composites having nanotubesarchitectures embedded therein are conducting, and sustain theconducting character over large percentages of strain imposed upon theflexible composite. Measurements of resistance as a function of tensileand compressive strains, with deformation and resistance measurementsconducted perpendicularly to the alignment of nanotubes in a compositesample are shown in FIGS. 11A and 11B. FIG. 11A shows the typicalvariation of the normalized composite resistance according to thisembodiment, under an applied tensile strain. The inset graph of FIG. 11Ashows a summary of the zero strain conditions before and after eachstrain cycle. As seen, the resistance scales generally linearly beyond asmall strain value (approximately 2.5%). The inset graph shows thechange in zero-strain resistance before a strain cycle as indicated bythe open markers in the graph, and after a strain cycle as indicated bythe solid markers in the graph. This summary generally shows that therewas an irreversible increase in normalized resistance of approximately10-15% after the first strain cycle, which thereafter stabilizes overmultiple strain cycles.

FIG. 11B shows the normalized resistance as a function of compression,with the values shown as a “log-log” plot showing the normalizedresistance during compressive strain cycles. The inset graph of FIG. 11Bagain shows the summary of the zero-strain resistance both before andafter each strain cycle in the testing. As can be seen in FIG. 11B, thenormalized resistance increases in general following a power-lawdependence on strain. The inset graph shows the device can detect verysmall changes in pressure of the compressive strain, such as for examplein the range of approximately 1000 N/m².

For these examples, a sample of composite material was provided having alength of 1-2 centimeters, a width of 1-2 millimeters, and a height ofapproximately 100 μm. Titanium wires were embedded into the compositematrix during curing of the polymer to obtain electrical contacts. Thezero-strain lateral resistivity of the composite material varied between1-10 Ω-cm for various samples, and increased monotonically for bothtensile and compressive strains. The aligned nanotube architectureprovides a lateral network of conducting fibers, which are connected toeach other, and provide a conducting path through the material. Tensileand compressional forces may change the contact area between neighboringfilaments to produce variances in accompanying electrical properties.For example, based upon the resistance characteristics from strainsimposed upon the composite material, applications of these structures asstrain and pressure/touch sensors is contemplated. Further, based uponthe conducting nature of the carbon nanotubes under strain, flexibleelectronic requirements may be provided, such as a flexible cathode foran integrated field-emission device (FED). The high aspect ratio of thecarbon nanotubes or other nanostructures and electrical conductingcharacteristics, would allow use in field emission technologies such asa field emission display devices.

As shown in FIG. 12, a flexible/plastic field-emission display may beprovided using the composites according to the invention. Carbonnanotube structures may be patterned on a rigid glass substrate, withdifferent colors obtained using phosphorous technology, The schemes oftransferring the patterns into and onto a polymer surface can then beused to prepare CNT cathodes wherein a bundle of carbon nanotubes 30 isembedded within insulating polymer 32 according to the invention. Anelectron transporting layer 34 is provided on top of the CNT bundle,with a layer of phosphorous or LED polymers 36 disposed on the top ofthe composite material. A protective plastic layer 38 may be provided,with electrical contacts made to the cathode assembly. The flexibleinterconnect layer 34 may be integrally formed into the assembly ifdesired, and the polymer matrix itself may incorporate phosphorous-basedmaterials or LED polymers to reduce the need for providing suchmaterials otherwise. Patterns may be disposed on the polymer substratesusing soft lithography or other suitable techniques. As it is possibleto control the roughness, etching and length of the carbon nanotubes,depending upon the display application, the cathode may be optimized forefficient field-emission. In addition, the chemistry may be optimized toobtain high efficiency of light emission. It is contemplated thatflexible displays according to the invention may be used for a varietyof applications, including, high definition displays for television,portable newspapers and magazines, head gear for military orentertainment applications, cell or smart phones, PDA's and many otherapplications.

As an example, testing of field-emission properties was performed onsamples of composite materials according to the invention. As shown inFIG. 6, a patterned MUVNT-PDMS composite is formed in a cylindricalshape, such as having a diameter of approximately 500 μm. It has beenfound that adjusting the quantity of PDMS in fabricating the compositeallow films to be produced with few or no exposed nanotubes on the topsurface of the composite, while the bottom surface of the pattern, wherethe ends of the nanotubes were completely exposed, was coated with aTi/Au material and fixed to a metal electrode using a conducting silverpaint or the like, thereby forming a composite cathode. A metal anodewith an adjustable separation distance was positioned parallel to thetop surface of the MWNT-PDMS composite, and the gap therebetween wasadjustable. As a possible preconditioning step, high currents may be runthrough the cathode/anode arrangement, to cause any long or entangledmasses of nanotubes to be burnt off while retaining desired length andseparation of nanotubes on the top surface of the film composite.Field-emission measurements were performed under a vacuum ofapproximately 5×10⁻⁴ Torr. When the effective electric field around ananotube tip exposed on the surface is large enough to overcome the workfunction of the nanotube (typically estimated at about 5 eV for carbonnanotubes), field emission will occur. The emitted current follows aknown mechanism, called the Fowler-Nordheim mechanism, where the currentdensity is approximately related to the effective field through theequation:

J _(FN)=(e ³ F ²/8phf)exp[−(8pv(2m)/3he)(f ^(3/2) /F)],

wherein F is the effective electric field seen by the emitting region,and f is the work function of the nanotube. If the separation distanceis d, then the field-enhancement factor, β, is the ratio between theeffective field and the applied field, set forth as:

β=F/V/d),

where V is the applied voltage across the device electrodes. Further, ifthe effective surface area of the emission is denoted a, then themeasured current is given by:

I=aJ_(FN).

This expression can be written as:

ln(I/V ²)=ln C₁−C₂/V, where, C₁=(e³/8phd²f)β² _(a) andC₂=(8pdf^(3/2)v(2m)/3he)(1/β).

As shown in FIG. 13, a Fowler—Nordheim (FN) plot shows the relationshipof ln (I/V²) versus 1/V. The characteristics of two samples devices areshown, with the inset graph showing the emission current for appliedvoltage for each sample. From the Fowler-Nordheim plot of FIG. 13, itcan be seen that the emission characteristics of the system generallyfollows the FN equation over a broad range of applied voltages, withslight deviations at the lowest and highest bias values. Such deviationsmay be associated with instrument insensitivity or at the high valuepossibly from an enhanced field current due to various factors. From theslope of the FN plots as shown in FIG. 13, a field-enhancement factor,β, of approximately 8,000 was obtained for Device 1, with a better valueof approximately 19,100 for Device 2. Other device properties obtainedfrom the FN plots are set forth in the following Table 1.

TABLE 1 Device Properties Obtained From the Fowler - Nordheim PlotsEnhancement Device Factor, β E_(to) (V/μm) Comments on Current Density 18000 0.87 1 mA/cm² @ 2.16 V/μm 2 19100 0.5 1 mA/cm² @ 0.76 V/μm

As seen therein, the turn-on fields, E_(to), were calculated for thedevices, with the values listed in Table 1 indicating high fieldemission. A nominal current density of 1 mA/cm² over the entire topsurface of a nanotube, was achieved easily at threshold fields of 2.16and 0.76 V/μm for the samples of devices made according to theinvention. These values can be reduced by patterning smaller-diameterpillars. Further electrical isolation of the emitting nanotubes fromneighboring nanotubes may be provided by use of a dielectric material orinsulator therebetween to improve field emission characteristics. As anexample, in the samples of field emitters according to the invention,the devices formed had very few tips exposed above the PDMS surface, andthose nanotubes that were exposed, had an exposed length ofapproximately 2 to 3 μm, while being separated by distances of similaror larger lengths. This arrangement was found to decrease mutualscreening of the electric fields produced by the nanotubes, and provideslarge field-enhancement factors with low turn-on fields. The stabilityprovided by embedding the nanotubes within the PDMS matrix preventsmovement of the nanotubes with respect to the cathode during high-fieldoperations, providing a well functioning and durable device. For examplesub-1 V/μm turn-on fields and threshold fields of a few volts permicrometer may be provided, while remaining stable, flexible andtransferable. Such field effect transmission devices may be used forvarious portable electronic and electromechanical devices or otherapplications.

A field emission display according to the invention may then be formedusing the flexible FED as shown in FIG. 12 as an example. As thecomposite systems are shown to have very efficient electron emission,this may be used to provide a large screen display based on FEDtechnology. Carbon nanostructures on a flexible plastic substrate canprovide significant advantages in manufacturing of flexible displays forvarious applications as previously noted.

In another application according to the invention, a bioactive orbiocompatible coating may be formed using nanotubes disposed in apolymer matrix. Such a coating may be formed integrated with a polymerand formed to simulate macroscopic objects such as synthetic bloodvessels, stints, membranes for dialysis, and other components, which maybe exposed to blood or other biological materials. As shown in FIG. 13,a method of forming carbon nanotubes on the inside of a polymer/nanotubecomposite capillary is shown, wherein a glass rod or other suitablesubstrate 40 has carbon nanotubes 42 grown over its exterior surface.The rod with carbon nanotube fibers grown thereon may then be disposedin a larger diameter capillary 44. The annulus surrounding the glass rod40 with carbon nanotubes 42 disposed thereon is then filled with asuitable monomer or other polymer precursor material 46, andpolymerization is accomplished in any suitable manner, such as by theapplication of heat or the like. The polymer layer 46 is then disposedcompletely around the glass rod 40 and carbon nanotubes 42 formedthereon, thereby embedding the nanotubes in the polymer matrix. Afterpolymerization, the glass rod 40 can be removed and polymeric tubes withCNT embedded therein are formed. In an example, a desired length of theembedded nanotubes are selectively exposed so as to enable contacting abiological fluid disposed or flowing through the capillary. It is alsopossible to functionalize the inside of the capillaries with fluorinatedmonolayers to simplify the removal of the glass rod from a finishedtube. Further, it is possible to provide additional chemical propertiesby use of different monomers and cross-linking groups to prepareelastomers having desired mechanical strengths or other properties. Thecarbon nanotube surface exposed on the interior of the capillary mayhave various beneficial applications, such as a coating to preventinflammatory response, or as a stimuli for cell growth using electricalcurrents for tissue engineering. Although the formation of a syntheticblood vessel or capillary is shown, a similar approach may be used forproviding carbon nanotube architectures in any shape which needs to bereplicated for biological applications. The flexibility of the use ofvarious polymer materials, as well as the ability to combine chemicalcharacteristics in association with the polymer matrix and embeddednanotube structure, provides a great amount of flexibility in adaptingthe composites for various applications.

In yet another embodiment of the invention, a method for producing asuperhydrophobic and/or self-cleaning carbon nanotube tape or otherproduct is provided. The method includes having micro-patternedvertically aligned carbon nanotubes grown on a silicon substrate withsilicon dioxide. Photolithography (or other suitable method) is used todeposit a catalyst layer in patterns, for example square patterns, of 50and 500 μm on a silicon oxide wafer. The catalyst layer includes a layerof aluminum about 10 nm in thickness, which acts as a buffer layer, andan iron catalyst layer about 1.5 nm in thickness, which forms nanosizeparticles for catalytic growth of the superhydrophobic carbon nanotubes.Ethylene, at a flow rate of 50-150 standard cubic centimeters/minute(cc/min), may be used as a carbon source, and the reaction is carriedout at a temperature of about 750° C. An Ar/H₂ gas mixture (15% H₂) witha flow rate of about 1300 cc/min is used as the buffer gas. Water vaporwith a dew point of −20° C. is introduced in the reaction furnace byAr/H₂ flow during the superhydrophobic carbon nanotube growth. Thegrowth time is about three minutes, and the length of thesuperhydrophobic carbon nanotubes is about 100 μm. The average diameterof the carbon nanotubes was measured to be about 8 nm (2-5 walls),forming even smaller (8-10 nm) carbon nanostructures in the biggerpatterns. The smaller patterns may be within the range of 1 to 40 nm, ormore particularly within 4 to 20 nm for example. An adhesive, forexample a flexible substrate such as a polymer tape or film with a tackycoating on one side, may be pressed against the top surface of thesuperhydrophobic carbon nanotubes. Upon peeling, the carbon nanotubesare transferred from the silicon substrate to the adhesive. FIGS. 15A-Cshow SEM images of the superhydrophobic carbon nanotubes of about 50 and500 μm patterns.

In order to show the superhydrophobic properties of the carbonnanotubes, a 10 μL water droplet on a 250 μm pattern is shown in FIG.16. The static contact angle of 155±3° was observed indicating thesuperhydrophobic properties of the hierarchical structure of the carbonnanotubes. Similar contact angles were also observed even after exposingthe samples with water after multiple exposures.

It has been shown that as the superhydrophobic carbon nanotubes are heldby the adhesive, the individual carbon nanotube structures do notcollapse due to capillary forces as seen in FIG. 17. If desired, surfacetreatment of the carbon nanostructures may provide predeterminedchemical properties, such as coating the nanostructures with hydrophobicfluorinated polymers or providing nonaligned carbon nanotubes, toprovide high water contact angles and superhydrophobic and self-cleaningcharacteristics. However, coating the carbon nanotubes with fluorinatedpolymers or providing nonaligned carbon nanotubes may not be desired.The use of the adhesive in the method for preparing the superhydrophobiccarbon nanotube tape or material may eliminate the use of fluorinatedcoatings or other non-wetting materials on the carbon nanotubes, whileallowing them to maintain their superhydrophobic and self-cleaningproperties.

In order to demonstrate the self-cleaning ability of the syntheticsuperhydrophobic carbon nanotubes, as seen in FIGS. 18A-D, the tapes ormaterials were soiled with silica particles ranging from 1-100 μm insize. The silica particles are used to represent dust. When rinsed withwater, the water droplets roll off the superhydrophobic carbon nanotubetape or materials carrying with them most of the silica particles. Asubstantially clean surface of the tape or material is shown in FIG.18C. There was substantially no macroscopic damage of thesuperhydrophobic carbon nanotube pillars after rinsing with water.

The self-cleaning properties of the superhydrophobic carbon nanotubetapes or materials were also tested by contact mechanics, to understandthe self-cleaning process of a gecko's feet. After a few contacts withmica or a glass substrate, it was observed that the majority of theparticles are transferred to the mica or glass surface. The effect iseven more clearly seen when a small vibration is applied. Most of thesilica particles fall off of the surface, with only a few small-sizeparticles remaining on the surface and in between the pillars of thecarbon nanotubes. The mechanically cleaned superhydrophobic carbonnanotube tape or material is shown in FIG. 18D.

Although the superhydrophobic carbon nanotube tapes or materials appearto be substantially free of silica particles after cleaning with wateror contact mechanics, the measurement of shear resistance was conductedin order to determine the degree in which the soiled samples regain theshear resistance after cleaning when compared with the pristine samplesbefore soiling with silica particles. The results show that the shearstress of the cleaned samples was measured to be about 60-90% of thosefor the pristine, unsoiled samples. The measurements were conducted witha shear device as shown in FIG. 19A. The superhydrophobic carbonnanotube tapes or materials, measuring about 0.16 cm² in area, werepressed against a clean mica surface with a preload of about 25-50N/cm². The tapes or materials were then pulled at a velocity of about400 μm/s.

The measured shear forces for the pristine sample, the sample soiled bysilica particles and subsequently cleaned by water, and the samplesoiled by silica particles and cleaned by mechanical vibration are shownin FIG. 19B. It can be seen that the shear stress for the pristinesample was around 5 N. When the tape or material was dusted and cleanedusing vibrations, it regained about 90% of the shear stress. Theslightly lower shear forces are due to smaller silica particlesremaining on the surface and in the spacing between the carbon nanotubeareas. The recovery of shear resistance in the superhydrophobic carbonnanotube tapes or materials is revealing since the live gecko only showsa recovery of about 50% of the shear stress after testing. When the tapeor material was dusted and cleaned by water, it regained about 60% ofthe shear stress. The lower efficiency of the water cleaned samples isthought to be due to the formation of micro-cracks within the carbonnanotube areas during drying of the water. Similar results are providedfor single-walled carbon nanotubes (SWNT) as seen in FIG. 20. Thereforethe method of forming a self-cleaning carbon nanotube composite materialmay use nanometer-size structures formed by one or more individualnanostructures. These maybe single or multi-walled nanotubes or othernanostructures depending on the strength requirements of theself-cleaning adhesive. In FIG. 20, the measurements on single wallednanotubes are shown for different areas of non-patterned single wallednanotubes. The force per unit area is falling with respect to increasingcontact area of a tape as would be expected from a non-patternednanotube adhesive structure using SWNT (similar to the behavior seen formulti-walled nanotubes).

In order to increase the shear resistance measurements, it is believedthat a reduction in the spacing between the carbon nanotube areas so asnot to provide a space for the smaller silica particles to penetratebetween the individual carbon nanotube pillars may be performed. Inaddition, it is envisioned that optimizing the adhesive that holds thecarbon nanotube pillars at the base may reduce the micro-cracksdeveloped during the water drying process.

While the invention has been illustrated and described in detail in theforegoing drawings and description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly illustrative embodiments thereof have been shown and described andthat all changes and modifications that come within the spirit of theinvention are desired to be protected. Additional features of theinvention will become apparent to those skilled in the art uponconsideration of the description. Modifications may be made withoutdeparting from the spirit and scope of the invention.

1. A self-cleaning carbon nanotube composite material comprising, asubstrate; an adhesive coating on at least a portion of the substrate; aplurality of carbon nanostructures formed into a predeterminedarchitecture, each of the plurality of nanostructures having asubstantially predetermined width and length, and the architecture ofthe plurality of nanostructures defining at least one orientation for aplurality of nanostructures, and defining the approximate spacingbetween nanostructures and/or groups of nanostructures, the carbonnanostructures architecture being at least partially adhered to theadhesive coating on the substrate in a manner that the architecture isstabilized in the predetermined architecture, wherein the carbonnanostructures architecture renders the composite materialsuperhydrophobic.
 2. The self-cleaning carbon nanotube compositematerial of claim 1, wherein the predetermined architecture comprises aplurality of nanostructures which include the combination of micron-sizestructures formed by bundles or groups of nanostructures, andnanometer-size structures formed by one or more individualnanostructures.
 3. The self-cleaning carbon nanotube composite materialof claim 2, wherein the micron-size structures formed by groups ofnanostructures have a width in the range from about 50 μm to about 500μm.
 4. The self-cleaning carbon nanotube composite material of claim 3,wherein the micron-size structures formed by groups of nanostructuresare spaced from one anther a distance varying in a range from about 5 μmto about 500 μm.
 5. The carbon nanotube/polymer composite material ofclaim 2, wherein the micron-size structures formed by one or moreindividual nanostructures are about 1 to 40 nm in width.
 6. Theself-cleaning carbon nanotube composite material of claim 2, wherein theheight of exposed nanostructures is in the range from about 10 μm toabout 1000 μm.
 7. The self-cleaning carbon nanotube composite materialof claim 1 substantially devoid of fluorinated coatings or othernon-wetting materials.
 8. The self-cleaning carbon nanotube compositematerial of claim 1, wherein the composite material is cleanable withwater and mechanical vibrations.
 9. The self-cleaning carbon nanotubecomposite material of claim 1, wherein a cleaned composite material hasa measured shear stress in the range from about 60% to about 90% of acomposite material prior to soiling.
 10. The self-cleaning carbonnanotube composite material of claim 9, wherein a water-cleanedcomposite material has a measured shear stress of about 60%.
 11. Theself-cleaning carbon nanotube composite material of claim 9, wherein amechanically cleaned composite material has a measured shear stress ofabout 90%.
 12. A method of forming a self-cleaning carbon nanotubecomposite material, the method comprising the steps of: providing asubstrate having a predetermined configuration, providing a plurality ofcarbon nanostructures formed in a substantially predeterminedarchitecture supported on the substrate, at least partially embeddingthe plurality of carbon nanostructures in an adhesive in a manner tostabilize the predetermined nanostructure architecture at leastpartially therein, wherein the arrangement of the predeterminednanostructure architecture renders the composite materialsuperhydrophobic.
 13. The method of forming a self-cleaning carbonnanotube composite material of claim 12, wherein the predeterminedarchitecture comprises a plurality of nanostructures which include thecombination of micron-size structures formed by bundles or groups ofnanostructures, and nanometer-size structures formed by one or moreindividual nanostructures.
 14. The method of forming a self-cleaningcarbon nanotube composite material of claim 13, wherein the micron-sizestructures formed by bundles or groups of nanostructures vary in a rangefrom about 50 μm to about 500 μm in width.
 15. The method of forming aself-cleaning carbon nanotube composite material of claim 13, whereinthe micron-size structures formed by bundles or groups of nanostructuresare spaced from one anther a distance varying in a range from about 5 μmto about 500 μm.
 16. The method of forming a self-cleaning carbonnanotube composite material of claim 13, wherein the nanometer-sizestructures formed by one or more individual nanostructures are about 1to 40 nanometers in width.
 17. The method of forming a self-cleaningcarbon nanotube composite material of claim 13, wherein the height ofexposed nanostructures is in a range from about 10 μm to about 1000 μm.18. The method of forming a self-cleaning carbon nanotube compositematerial of claim 12, wherein the step of providing the plurality ofcarbon nanostructures comprises providing a plurality of carbonnanotubes having at least one substantially predetermined width andlength.
 19. The method of forming a self-cleaning carbon nanotubecomposite material of claim 12, further comprising the step of providingthe plurality of carbon nanostructures to have at least onesubstantially predetermined orientation for a plurality ofnanostructures.
 20. The method of forming a self-cleaning carbonnanotube composite material of claim 12, further comprising the step ofproviding the plurality of carbon nanostructures to have a predeterminedspacing between nanostructures and/or groups of nanostructures.
 21. Themethod of forming a self-cleaning carbon nanotube composite material ofclaim 12, further comprising the step of embedding only a portion of theplurality of carbon nanostructures to have at least a portion thereofextending from the surface of the adhesive.
 22. The method of forming aself-cleaning carbon nanotube composite material of claim 12, furthercomprising the step of providing the adhesive to have at least onesubstantially predetermined thickness.
 23. The method of forming aself-cleaning carbon nanotube composite material of claim 12, whereinthe adhesive is a polymer tape.
 24. A method of forming a self-cleaningcarbon nanotube composite material, comprising the steps of: forming acomposite material having a predetermined architecture of substantiallyvertically aligned carbon nanostructures embedded in an adhesive, theadhesive having a predetermined thickness, wherein the spacing betweenthe substantially vertically aligned carbon nanostructures issubstantially predetermined and the carbon nanostructures extend from atleast one surface of the adhesive a substantially predetermined amount,such that the exposed carbon nanostructures renders the compositematerial superhydrophobic.
 25. The method of forming a self-cleaningcarbon nanotube composite material of claim 24, wherein thepredetermined architecture comprises a plurality of nanostructures whichinclude the combination of micron-size structures formed by bundles orgroups of nanostructures, and nanometer-size structures formed by one ormore individual nanostructures.
 26. The method of forming aself-cleaning carbon nanotube composite material of claim 25, whereinthe micron-size structures formed by bundles or groups of nanostructuresvary in a range from about 50 μm to about 500 μm in width.
 27. Themethod of forming a self-cleaning carbon nanotube composite material ofclaim 25, wherein the micron-size structures formed by bundles or groupsof nanostructures are spaced from one anther a distance varying in arange from about 5 μm to about 500 μm.
 28. The method of forming aself-cleaning carbon nanotube composite material of claim 25, whereinthe nanometer-size structures formed by one or more individualnanostructures are about 1 to 40 nanometers in width.
 29. The method offorming a self-cleaning carbon nanotube composite material of claim 25,wherein the height of exposed nanostructures is in the range from about10 μm to about 1000 μm.
 30. The method of forming a self-cleaning carbonnanotube composite material of claim 24, wherein the carbonnanostructures are carbon nanotubes.
 31. The method of forming aself-cleaning carbon nanotube composite material of claim 24, whereinthe carbon nanostructures are carbon nanofibers.